Titanium dioxide (TiO2, also know as titania) has been widely studied because of its potential photocatalytic applications. Titanium dioxide only absorbs ultraviolet (UV) radiation. When UV light is illuminated on titanium dioxide, electron-hole pairs are generated. Electrons are generated in the conduction band and holes are generated in the valence band. The electron and hole pairs reduce and oxidize, respectively, adsorbates on the surface of the titanium dioxide, producing radical species such as OH− and O2−. Such radicals may decompose certain organic compounds. As a result, titanium dioxide coatings have found use in antimicrobial and self-cleaning coatings, for example on windows.
To activate the titanium dioxide to photogenerate these electron-hole pairs (i.e. photocatalytic activity), and thus to provide the titanium dioxide with antimicrobial and/or self-cleaning properties, titanium dioxide must be regularly dosed with photons of energy greater than or equal to about 3.0 eV (i.e., radiation having a wavelength less than about 413 nm). Depending on variables such as the structure, ingredients, and texture of titanium dioxide coatings, for example, dosing may takes several hours, such as, for example, 6 hours or more. Antimicrobial titanium dioxide coatings, therefore, must generally be exposed to UV radiation for at least 6 hours before achieving the full photocatalytic effect.
Efforts have been made to extend the energy absorption of titanium dioxide to visible light and to improve the photocatalytic activity of titanium dioxide. For example, foreign metallic elements such as silver can be added. This may, for example, aid electron-hole separation as the silver can serve as an electron trap, and can facilitate electron excitation by creating a local electric field. The use of silver, however, requires tempering the coating in a nitrogen environment to prevent the silver from oxidizing. Thus, adding silver to titanium dioxide coatings on a large scale is not a viable option due to the high costs.
Titanium dioxide also has been shown to exhibit highly hydrophilic properties when exposed to UV radiation. Such hydrophilicity may be beneficial in certain embodiments, such as, for example, certain coating embodiments. Without wishing to be limited in theory, it is believed that the photoinduced hydrophilicity is a result of photocatalytic splitting of water by the mechanism of the photocatalytic activity of the titanium dioxide, i.e., by the photogenerated electron-hole pairs. When exposed to UV radiation, the water contact angle of titanium dioxide coatings approaches 0°, i.e., superhydrophilicity.
Current coating methods involving titanium dioxide often result in a disadvantageous loss of hydrophilicity and/or photocatalytic activity such as antimicrobial and/or self-cleaning properties of the titanium dioxide. This may be due to formation of different phases of the titanium dioxide during the coating process. For example, anatase titanium dioxide typically transforms to rutile phase titanium dioxide when heated at temperatures which may be used during the coating process, such as temperatures greater than 600° C., such as may be used during the coating process. The rutile phase has less desirable surface coating properties than the anatase phase, such as, for example, less desirable hydrophilicity and antimicrobial and/or self-cleaning properties.
There is thus a long-felt need in the industry for methods for forming a titanium dioxide coating having increased photocatalytic activity such as antimicrobial and/or self-cleaning properties and/or hydrophilicity, and/or a reduced dosing time. The invention described herein may, in some embodiments, solve some or all of these needs.